Secure and Efficient Interoperability on Arbitrum with zkBridge

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zkBridge presents a highly secure framework for seamless message transmission and asset bridging across multiple L1 and L2 networks. Utilizing zkBridge, state transitions on the source chain are verified on the destination chain in a trustless manner, eliminating dependence on the security protocols of any third-party intermediaries. This is facilitated by the incorporation of zero-knowledge proofs, ensuring robust security independent of external variables and notably minimizing the costs associated with on-chain verifications. Furthermore, zkBridge operates on a decentralized network dedicated to proof generation, welcoming any node to participate freely. These nodes undertake responsibilities for relaying block headers and proof generation. Upon successful execution, they are entitled to rewards. Integral to zkBridge is its foundational proof generation mechanism, deVirgo. Recognized for its exceptional speed in the realm of zero-knowledge proof systems, deVirgo employs parallel computing to expedite the process of block header proof generation. This efficiency is pivotal, allowing the pace of proof generation to align closely with that of block production, enhancing the overall reliability and performance of cross-chain interoperability.

While optimistic rollups such as Arbitrum provide a high transaction throughput, they represent a formidable challenge to cross-chain interoperability. If we hope zkBridge to maintain the same security assumptions as Arbitrum, then zkBridge should track finalized states from L2-to-L1 withdrawal transactions and L2-to-L1 messages in order to generate the state proofs for L2, and also verify the Ethereum full consensus with zkBridge for the settlement L1 blockchain (i.e., Ethereum) for Arbitrum.

The above approach for building zkBridge from Arbitrum first triggers an L2-to-L1 withdrawal transaction or a L2-to-L1 messaging, and then waits for the fraud proof to finalize (could be as long as 7 days for Arbitrum). zkBridge needs to prove both the consensus of the settlement chain (Ethereum full PoS) and the finalized state for the L2-to-L1 transaction, so that the rollup block written on the settlement chain can be used. While this is secure, it creates a high latency for cross-chain applications.

To address the delay issue in layer-2 bridging with current optimistic rollups to achieve both fast finality and maximum security, we introduce a solution leveraging “auxiliary validation network”.

zkBridge fast finality for Arbitrum powered by deVirgo

We introduce our solution with a decentralized auxiliary validation network for the fast finality of cross-chain transactions from Arbitrum to other networks. The auxiliary validation network forms an external group, attesting to the state of Arbitrum, to achieve security levels comparable to Arbitrum’s fraud proof challenges. By introducing external validators of Arbitrum with decentralized trust, the auxiliary validation network rapidly simulates the fraud-proof process by replaying the transactions of the Arbitrum sequencer, achieving strong security nearly equivalent to the 7-day challenge window. Furthermore, we can incorporate the crypto-economic security from staking and restaking protocols (e.g., EigenLayer) in the future, which helps to achieve stronger security guarantees with both decentralized and economic trust.

While the proposed system architecture may seem straightforward, real-world implementation comes with its technical challenges. One such challenge is determining the optimal size for the auxiliary validation network. A larger committee inherently augments the security framework of rollup bridges. However, increasing the size brings challenges in terms of validation speed and scalability. Furthermore, there’s the significant task of validating a vast number of signatures and generating their proof in a tight time window. The challenge intensifies when dealing with hundreds of signatures, necessitating a high-speed proof system.

This is where our proprietary proof system, deVirgo, stands out. At Polyhedra Network, we have achieved a milestone with deVirgo, validating over 32,000 BLS signatures within 8 seconds, which is deployed for proving Ethereum full PoS consensus. This efficiency allows us to generate proof for all signatures in the auxiliary validation network in just a matter of seconds.

Leveraging this innovative proof system, our fast finality zkBridge on Arbitrum not only benefits from swift finality but also boasts robust security on par with L1 bridges. This combination is optimal for real-world bridge deployments, striking a balance between speed and security. In the future, our solution can be integrated with Arbitrum Stylus to further enable flexibility and compatibility for fast finality.

Beyond, Arbitrum Orbit launches new chains in its ecosystem. Integrating zkBridge and the auxiliary validation network ensures quick finality for these chains and apps, offering Ethereum-like security, superior scalability, rapid validation, and enhanced interoperability.

Integrating zkBridge with Arbitrum

Drawing inspiration from our original zkBridge design, we have deployed the fast finality zkBridge and decentralized auxiliary validation network for both Arbitrum One and Arbitrum Nova. This network is dedicated to validating signatures and then presenting the relevant proof on the destination blockchain. There are two components of building zkBridge for Arbitrum as the sender chain with the proving network:

  1. zkBridge verifies the state proofs from the auxiliary validation network after the Ethereum equivalent finality state on Arbitrum.
  2. zkBridge verifies the Ethereum full PoS consensus for the settlement L1 of Arbitrum.

To further reduce the proof verification cost, we can support the batch proof for multiple batches in Arbitrum, which helps to save the gas cost on the receiver chain for zkBridge proof verification.

Designing multiple proof scheme for Arbitrum

Nevertheless, while developing the zkBridge for optimistic rollups, an innovative multiple proof scheme for Arbitrum and other optimistic rollups came to light. This optimized scheme increases the efficiency, interoperability and scalability while maintaining the security of optimistic rollups. Below is how this newly conceived scheme functions:

  1. Proof Submission: The generator, instead of following traditional routes, directly submits the proof to a smart contract on Layer 1 (L1).
  2. Automated Validation: This L1 smart contract is designed to auto validate the submitted proof. The automated process eliminates manual verifications and associated delays.
  3. Instantaneous Finalization: Once the proof is successfully validated, Layer 1 instantly finalizes the state update on Layer 2 (L2).

This scheme offers transformative advantages when integrating with Arbitrum:

  1. Reduced Finality Time: A notable benefit is the significant reduction in the finality time for current optimistic rollups. The elimination of manual checks and the automation of proof validation processes accelerate transaction finalizations.
  2. Efficiency and Scalability: By automating the proof validation, the scheme paves the way for a more scalable solution, potentially handling a larger number of rollup transactions without compromising on efficiency.

Building zkBridge on Arbitrum Stylus

Arbitrum Stylus is a new design which enables an innovative way to write smart contracts by introducing a second, co-equal virtual machine that is fully interoperable with the EVM. The potential integration of zkBridge with Arbitrum Stylus heralds significant improvements in the realm of smart contracts, particularly in the nuanced area of zero-knowledge proofs. This synergy is poised to simplify the process of smart contract creation, streamlining the authentication of a diverse range of proofs via the innovative mechanisms of zkBridge.

One of the most notable enhancements expected from this fusion is the boost in operational efficiency. Arbitrum Stylus, celebrated for its computational prowess and optimal memory usage, will facilitate a swifter process of on-chain proof validation. This not only improves the speed but also broadens compatibility, accommodating a variety of programming languages and systems, thus extending its reach beyond the conventional confines.

Additionally, the integration aims to cultivate a more user-friendly experience by instituting easy-to-navigate zero-knowledge proof protocols within the Stylus ecosystem. This strategic move is designed to demystify the sophisticated proof systems, making them more accessible to a wider user base and fostering an environment of inclusivity. In terms of security and confidentiality, the collaborative effort is set to break new ground. It will enable the crafting of private contracts, a revolutionary feature that safeguards critical developer information, reinforcing the pillars of privacy in contractual engagements.

Moreover, the convergence of zkBridge’s auxiliary validation network with Stylus is a strategic step towards bolstering the efficacy and security of zkp-based oracles. This enhancement is crucial, given the pivotal role of these oracles in securing decentralized finance transactions among other key applications. All things considered, this integration is set to redefine the landscape of smart contract interactions across multiple facets, namely efficiency, user accessibility, privacy, and the robustness of zk oracles.

Conclusion

zkBridge is a pivotal innovation in addressing the challenges of cross-chain interoperability, particularly within the context of Arbitrum. By adopting this strategy, we can forge a new bridge for Layer 2 optimistic rollups, delivering expedited finality and streamlining inter-blockchain interoperability using zkBridge. Our design offers unparalleled security and high efficiency. The synergy between Arbitrum and zkBridge not only enhances user experiences in cross-chain interactions but also sets the stage for a more integrated and secure digital blockchain future.

By integrating zkBridge into the Arbitrum framework, the combined system addresses several pain points traditionally associated with blockchain interfaces, including transaction speeds, proof validations, and overall system interoperability. The result is a symbiotic digital infrastructure that enhances operational aspects for users and lays the groundwork for advanced, secure, and efficient blockchain ecosystems in the future. This forward-thinking approach sets a new standard for how blockchain networks can collaborate, driving innovation and user-centric solutions in the digital currency space.